首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1302605篇
  免费   114660篇
  国内免费   1390篇
  1418655篇
  2021年   17724篇
  2020年   12562篇
  2019年   16112篇
  2018年   17411篇
  2017年   16114篇
  2016年   27710篇
  2015年   42142篇
  2014年   50143篇
  2013年   76356篇
  2012年   33969篇
  2011年   21776篇
  2010年   42557篇
  2009年   44294篇
  2008年   21132篇
  2007年   18810篇
  2006年   24759篇
  2005年   25954篇
  2004年   25089篇
  2003年   22798篇
  2002年   20935篇
  2001年   34302篇
  2000年   31621篇
  1999年   31465篇
  1998年   25289篇
  1997年   25105篇
  1996年   24653篇
  1995年   22730篇
  1994年   22610篇
  1993年   21700篇
  1992年   26999篇
  1991年   25464篇
  1990年   23850篇
  1989年   24767篇
  1988年   22795篇
  1987年   21388篇
  1986年   20188篇
  1985年   22028篇
  1984年   21926篇
  1983年   19401篇
  1982年   20062篇
  1981年   19407篇
  1980年   18030篇
  1979年   17757篇
  1978年   16967篇
  1977年   16296篇
  1976年   15412篇
  1975年   14787篇
  1974年   15028篇
  1973年   15463篇
  1972年   12884篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
971.
972.
Nucleotide sequence of pheW; a third gene for E. coli tRNAPhe.   总被引:2,自引:0,他引:2       下载免费PDF全文
  相似文献   
973.
Abstract: Pharmacologically active agents were employed to study the mechanisms that control the reduction in levels of acetyl-coA: arylamine N-acetyltransferase activity (NAT) (EC 2.3.1.5) in the rat pineal. Pretreatment of rats with phenoxybenzamine or phentolamine prevented the rapid light-mediated decrease in NAT activity, although pretreatment with yohimbine or atropine did not alter this effect of light. Administration of mecamylamine resulted in a rapid reduction in enzyme activity prior to light exposure. When clonidine was administered intraperitoneally to animals with elevated NAT levels, there was a rapid decrease in enzyme activity, mimicking the effects of light. However, intraperitoneal injections of norepinephrine, methoxamine and phenylephrine into similar groups of animals had no significant effect on enzyme acitivity. When clonidine and norepinephrine were administered intraventricularly, there was a rapid reduction in enzyme activity. On the other hand, intraventricular administration of phenylephrine did not result in reduced enzyme activity. Pretreatment of animals with phenoxybenzamine failed to block the reduction in NAT activity precipitated by low doses of clonidine. This clonidine-mediated reduction in enzyme activity was, however, blocked by yohimbine. When animals were simultaneously exposed to light and administered clonidine, the rapid reduction in NAT activity was affected only when animals were pretreated with both yohimbine and phenoxybenzamine. In contrast to the decrease in pineal NAT activity observed in in vivo preparations, incubation of pineals with clonidine in an organ culture system produced a moderate, but consistent, rise in enzyme activity. These results suggest that stimulation of a receptor with α-adrenergic characteristics mediates the reduction in NAT activity produced by light. Stimulation of yet a second adrenergic-like receptor appears to mediate a reduction in pineal NAT activity precipitated by clonidine. Our evidence suggests that one or both of these receptors are located within the central nervous system.  相似文献   
974.
Delays in the development of exploratory and locomotor behavior in neonatal male rats (up to 21 days of age) are shown to accrue as a consequence of low level lead exposure. Cross fostering experiments indicate that these delays are primarily due to prenatal exposure. These Pb induced behavioral modifications appear to be associated with the delays in synaptogenesis and biochemical development of the cerebral cortex reported previously (4, 18). A new behavioral bioassay for detecting delays in brain development is described.  相似文献   
975.
976.
977.
978.
979.
The nucleotide sequence of two cloned rat lens beta-crystallin cDNAs pRL beta B3-2 and pRL beta B1-3 has been determined. pRL beta B3-2 contains the complete coding information for a beta-crystallin, designated beta B3, of 210 amino acid residues. pRL beta B1-3 is incomplete at its 5' end; the 5' codogenic information which is not present in this cDNA clone was deduced from the cloned gene. pRL beta B1-3 codes for a beta-crystallin polypeptide, designated beta B1, whose full length is 247 amino acid residues. Considerable sequence homology is noted between the amino- and carboxy-terminal halves of each protein. The two rat beta-crystallins show a substantial sequence homology with each other (60%) as well as with the published sequences of rat gamma-crystallin (37%) and bovine and murine beta-crystallins (55 and 45%). All these proteins have a two-domain structure which, like the bovine gamma II-crystallin, might be folded into four remarkably similar protein motifs. Our data further indicate that the beta-crystallins can be subdivided into two groups which are evolutionarily related. Both groups are, although more distantly, also related to the gamma-crystallins.  相似文献   
980.
It has been shown that at late periods after immobilization stress there is an essential reduction in the cytostatic activity of CBA mice splenocytes detected by inhibition of mastocytoma P 815 cell proliferation in the test with 3H-uridine incorporation. The cytostatic activity returns to normal 5 to 14 days after stress. The changes in the cytolytic and cytostatic activity of mouse splenocytes following stress are correlated and a possible biological value of changes in the immunoassays for the host are discussed.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号